How much paint would I need to put a coat on a Jeep Grand Cherokee?

I've got an old 98 jeep grand Cherokee that I use fro towing my trailer once in a while. It's not worth a real paint job but it's embarrassing to drive with the paint all messed up like it is. I don't need it looking fancy, just better for a few more years. I've got the sprayers & everything and I was thinking about slapping a coat of paint matching the original color on there. I've painted lots of things just never a car.

How much paint should I order?

Thanks!